The top quark at LHC: status and prospects

1
The top quark at LHC status and prospects

• Marina Cobal-Grassmann
• Journee ATLAS France
• Londe Les Maure, 3-5 May, 2004

2
Motivations for Top Physics studies

• Top quark exists and will be produced abundantly!
• In SM top- and W-mass constrain Higgs mass
• Sensitivity through radiative corrections
• Scrutinize SM by precise determination top mass

• Beyond SM New Physics?
• Many heavy particles decay in tt
• Handle on new physics by detailed properties of
  top
• Experiment Top quark useful to calibrate the
  detector
• Beyond Top Top quarks will be a major source of
  background for almost every search for physics
  beyond the SM

Summer 2003 result
direct
EXCLUDED
indirect
3
What we know..
mH
No observable directly related to mH. However the
dependence can appear through radiative
corrections. ? tree level quantities changed
??, ?r f ln(mH/mW), mt2
The uncertainties on mt, mW are the dominating
ones in the electroweak fit
By making precision measurements (already
interesting per se) one can get information
on the missing parameter mH one can test the
validity of the Standard Model
4
Top mass Where we are
5
Near future of Mtop
Tevatron only (di-lepton events or leptonjet )
from W decays
Status of inputs (preliminary) mt(178.0 ? 2.7
(stat) ? 3.3 (syst)) GeV/c2 (latest Tevatron
updated combination RunI data) mt(175 ? 17
(stat) ? 8 (syst)) GeV/c2 (CDF di-leptons RunII
data) mt(17813-9 (stat) ? 7 (syst))
GeV/c2 (CDF leptonjets RunII data)

• Matter of statistics (also for the main
  systematics) and optimized use of the available
  information. Each experiment expects 500 b-tagged
  tt ljets events/fb ? DMtop 2-3 GeV/c2 for the
  Tevatron combined (2-4/fb)
• ?mt ? 2.5 GeV ?mW ? 30 MeV ? ?mH/mH ? 35

In 2009 (if upgrade is respected) from Tevatron
DMtop 1.5 GeV !!
6
What can we do at LHC?
process ?(pb) Events/s Events/y
bb 5?108 106 1013
Z?ee 1.5?103 3 107
W?l? (le,µ) 3?104 60 108
WW?e?X 6 10-2 105
tt 830 1.7 107
H(700 GeV/c2) 1 2?10-3 104
-
-
7
Top production at LHC

• Cross section determined to NLO precision
• Total ?NLO(tt) 834 100 pb
• Largest uncertainty from scale variation
• Compare to other production processes

90 gg10 qq
Low lumi
Process N/s N/year Total collected before start LHC
W? e? 15 108 104 LEP / 107 FNAL
Z? ee 1.5 107 107 LEP
tt 1 107 104 Tevatron
bb 106 1012-13 109 Belle/BaBar ?
H (130) 0.02 105 ?

• Top production cross section approximately 100x
  Tevatron

LHC is a top factory!
8
Top decay

• In the SM the top decays to Wb
• All decay channels investigated
• Using fast parameterized detector response
• Checks with detailed simulations

• Di-leptons (e/?)
• BR4.9 ? 0.4x106 ev/y
• No top reconstructed
• Clean sample
• Single Lepton (e/?)
• BR29.6 ? 2.5x106 ev/y
• One top reconstructed
• Clean sample
• Fully Hadronic
• BR45 ? 3.5x106 ev/y
• Two tops reconstructed
• Huge QCD background
• Large combinatorial bckgnd

9
MTop from leptonjet

• Golden channel
• Clean trigger from isolated lepton
• The reconstruction starts with the W mass
• different ways to pair the right jets to form the
  W
• jet energies calibrated using mW
• Important to tag the b-jets
• enormously reduces background (physics and
  combinatorial)
• clean up the reconstruction

• Br(tt?bbjjl?)30for electron muon

• Typical selection efficiency 5-10
• Isolated lepton PTgt20 GeV
• ETmissgt20 GeV
• 4 jets with ETgt40 GeV
• gt1 b-jet (?b?40, ?uds?10-3, ?c?10-2)

Background lt2 W/Zjets, WW/ZZ/WZ
10
Lepton jet reconstruct top

• Hadronic side
• W from jet pair with closest invariant mass
• to MW
• Require MW-Mjjlt20 GeV
• Assign a b-jet to the W to reconstruct Mtop
• Kinematic fit
• Using remaining lb-jet, the leptonic part is
  reconstructed
• ml?b -ltmjjbgt lt 35 GeV
• Kinematic fit to the tt hypothesis, using MW
  constraints

• Selection efficiency 5-10

11
Top mass systematics

• Method works
• Linear with input Mtop
• Largely independent on Top PT
• Biggest uncertainties
• Jet energy calibration
• FSR out of cone give large variations in mass
• B-fragmentation
• Verified with detailed detector simulation and
  realistic calibration

Source of uncertainty Hadronic ?Mtop (GeV) Fitted ?Mtop (GeV)
Light jet scale 0.9 0.2
b-jet scale 0.7 0.7
b-quark fragm 0.1 0.1
ISR 0.1 0.1
FSR 1.9 0.5
Comb bkg 0.4 0.1
Total 2.3 0.9
Challenge determine the mass of the top around
1 GeV accuracy in one year of LHC
12
Alternative mass determination

• Select high PT back-to-back top events
• Hemisphere separation (bckgnd reduction, much
  less combinatorial)
• Higher probability for jet overlapping
• Use the events where both Ws decay leptonically
  (Br5)
• Much cleaner environment
• Less information available from two ?s
• Use events where both Ws decay hadronically
  (Br45)
• Difficult jet environment
• Select PTgt200 GeV

Various methods all have different systematics
13
Jet scale calibration

• Calibration demands
• Ultimately jet energy scale calibrated within 1
• Uncertainty on b-jet scale dominates ?Mtop light
  jet scale constrained by mW
• At startup jet-energy scale known to lesser
  precision

Uncertainty On b-jet scale Hadronic
1 ? ?Mt 0.7 GeV 5 ? ?Mt
3.5 GeV 10 ? ?Mt 7.0 GeV
Uncertainty on light jet scale Hadronic
1 ? ?Mt lt 0.7 GeV 10 ?
?Mt 3 GeV
14
Alternative methods

• Determining Mtop from ?(tt)?
• huge statistics, totally different systematics
• But Theory uncertainty on the pdfs kills the
  idea
• 10 th. uncertainty ? ?mt ? 4 GeV
• Constraining the pdf would be very precious
• (up to a few might not be a dream !!!)

• Luminosity uncertainty then plays the game (5?)

Luminosity uncertainty then plays the game (5?)

• Continuous jet algorithm
• Reduce dependence on MC
• Reduce jet scale uncertainty
• Repeat analysis for many cone sizes ?R
• Sum all determined top massrobust estimator
  top-mass

15
Top mass from J/?

• Use exclusive b-decays with high mass products
  (J/?)
• Higher correlation with Mtop
• Clean reconstruction (background free)
• BR(tt?qqb??J/????) ? 5 10-5
• ? 30 ? 103 ev./100 fb-1 (need high lumi)

MlJ/?
Different systematics (almost no sensitivity to
FSR) Uncertainty on the b-quark fragmentation
function becomes the dominant error
M(J/?l)
M(J/?l)
Pttop
16
Search for resonances

• Many theoretical models include the existence of
  resonances decaying to top-topbar
• SM Higgs (but BR smaller with respect to the
  WW and ZZ decays)
• MSSM Higgs (H/A, if mH,mAgt2mt, BR(H/A?tt)1 for
  tanß1)
• Technicolor Models, strong ElectroWeak Symmetry
  Breaking, Topcolor, colorons production,

• Study of a resonance ? once known s?, G? and
  BR(??tt)
• Reconstruction efficiency for semileptonic
  channel
• 20 mtt400 GeV
• 15 mtt2 TeV

1.6 TeV resonance
Mtt
17
Couplings and decays

• Does the top quark behaves as expected in the SM?
• Yukawa coupling to Higgs from ttbarH events
• Electric charge
• Top spin polarization
• CP violation
• According to the SM
• Br(t ?Wb) ? 99.9, Br(t ? Ws) ? 0.1, Br(t ?
  Wd) ? 0.01
  (difficult to
  measure)
• Can probe t ?Wnon-b by measuring ratio of
  double b-tag to single b-tag
• Statistics more than sufficient to be sensitive
  to SM expectation for Br(t ? W s/d)
• need excellent understanding of b-tagging
  efficiency/purity

18
Rare decays FCNC

• In the SM the FCNC decays are highly suppressed
  (Brlt10-13-10-10)
• Any observation would be sign of new physics
• Sensitivity according to ATLAS and CMS studies
• t ? Zq (CDF Brlt0.137, ALEPH Brlt17, OPAL
  Brlt13.7)
• Reconstruct t ? Zq ? (ll-)j
• Sensitivity to Br(t ? Zq) 1 X 10-4 (100 fb-1)
• t ? ?q (CDF Brlt0.032)
• Sensitivity to Br(t ? ?q) 1 X 10-4 (100 fb-1)
  
• t ? gq
• Difficult identification because of the huge QCD
  bakground
• One looks for like-sign top production (ie. tt)
• Sensitivity to Br(t ? gq) 7 X 10-3 (100 fb-1)

19
Top Charge determination

• Can we establish Qtop2/3?
• Currently cannot exclude exotic possibility
  Qtop-4/3
• Assign the wrong W to the b-quark in top decays
• t?W-b with Qtop-4/3 instead of t?Wb with
  Qtop2/3 ?
• Technique
• Hard ? radiation from top quarks
• Radiative top production, pp?tt? cross section
  proportional to Q2top
• Radiative top decay, t?Wb?
• On-mass approach for decaying top two
  processes treated independently
• Matrix elements havebeen calculated and fed
  intoPythia MC

20
Top Charge determination

• Determine charge of b-jet andcombine with lepton
• Use di-lepton sample
• Investigate wrong combination b-jet charge and
  lepton charge
• Effective separation b and b-bar possible in
  first year LHC
• Study systematics in progress

• Yield of radiative photons allows to distinguish
  top charge

Q2/3 Q-4/3
pp?tt? 101 10 295 17
pp?tt t?Wb? 6.2 2.5 2.4 1.5
Total background 38 6 38 6
10 fb-1 One year low lumi
events
pT(?)
21
Top spin correlations

• In SM with Mtop?175 GeV, ?(t) ? 1.4 GeV ?QCD
• Top decays before hadronization, and so can study
  the decay of bare quark
• Substantial ttbar spin correlations predicted in
  pair production
• Can study polarization effects through helicity
  analysis of daughters
• Study with di-lepton events
• Correlation between helicity angles ? and
  ?-for e/? and e-/?-

ltCosT CosT-gt
ltCosT CosT-gt
With helicity correlation
No helicity correlation
22
Top spin correlations

• Also study spin correlations in hadronic decays
  (single lepton events)
• Least energetic jet from W decay ? 0.5
• Ratio between with and without correlations

30 fb-1
ltCosT CosT-gt

• Able to observe spin correlations in asymmetry
  C?
• 30 fb-1 of data
• 0,035 statistical error
• 0,028 systematic error
• 10? statistical significance for a non-zero
  value with 10 fb-1

23
Single top production
1) Determination of Vtb 2) Independent mass
measurement

• Three production mechanisms
• Main Background ?xBR(W?l?), le,µ
• tt s833 pb 246 pb
• Wbb s300 pb 66.7 pb
• Wjj s18103 pb 4103 pb

16.6
Wg fusion 24527 pb S.Willenbrock et al.,
Phys.Rev.D56, 5919
Wt 62.2 pb A.Belyaev, E.Boos,
Phys.Rev.D63, 034012
W 10.20.7 pb M.Smith et al., Phys.Rev.D54,
6696
-3. 7

• Direct determination of the tWb vertex (Vtb)
• Discriminants
• - Jet multiplicity (higher for Wt)
• More than one b-jet (increase W signal over W-
  gluon fusion)
• 2-jets mass distribution (mjj mW for the Wt
  signal only)

Wg 54.2 pb Wt 17.8 pb W 2.2 pb
24
Single top results

• Detector performance critical to observe signal
• Fake lepton rate
• b and fake rate id ?
• Reconstruction and vetoing of low energy jets
• Identification of forward jets
• Each of the processes have different systematic
  errors for Vtb and are sensitive to different new
  physics
• heavy W ? increase in the s-channel W
• FCNC gu ? t ? increase in the W-gluon fusion
  channel

• Signal unambiguous, after 30 fb-1
• Complementary methods to extract Vtb
• With 30 fb-1 of data, Vtb can be determined to
  -level or better(experimentally)

Process Signal Bckgnd S/B
Wg fusion 27k 8.5k 3.1
Wt 6.8k 30k 0.22
W 1.1k 2.4k 0.46
Process ?Vtb(stat) ?Vtb(theory)
Wg fusion 0.4 6
Wt 1.4 6
W 2.7 5
25
Undergoing analyses

• CP violation in top events (K. Martens,
  University of Toronto )
• Top spin polarization in di-lepton events (V.
  Simak et al., Prague)
• Top spin polarization in single lepton events (E.
  Monnier, P. Pralavorio, F. Hubaut, CPPM)
• Single top studies (M. Barisonzi, NIKHEF)
• Optimization of kinematic reconstruction in the
  single lepton channel (V. Kostioukhine,
  University of Genova)
• Commissioning studies (S. Bentvelsen, NIKHEF)
• New MC validation (S. Bentvelsen, E. Monnier, P.
  Pralavorio)
• Full simulation studies of detector effects (A.
  Etienvre, J. Schwindling, JP Meyer, Saclay)
• Full simulation studies of b-tagging (S. Moed,
  University of Geneva)
• Top mass and calibration studies (D. Pallin, F.
  Binet, Clermont-Ferrand)
• Ttbar resonances (E. Cogneras, Clermont-Ferrand)

26
What is left before the LHC starts?

• Cover topics still open cross section,
  couplings, exotic, resonances,
• Define a strategy for validation of the MC input
  models (e.g UE modeling and subtraction, jet
  fragmentation properties, jet energy profiles,
  b-fragmentation functions..)
• see M. Mangano talk at IFAE 2004
• Explore the effects of changing detector
  parameters in evaluating the top mass.
• Perform commissioning studies with top events
• Contribute to simulation validation

27
Commissioning the detectors

• Determination MTop in initial phase
• Use Golden plated leptonjet
• Selection
• Isolated lepton with PTgt20 GeV
• Exactly 4 jets (?R0.4) with PTgt40 GeV
• Reconstruction
• Select 3 jets with maximal resulting PT

Calibrating detector in comissioning phase Assume
pessimistic scenario -) No b-tagging -) No jet
calibration -) But Good lepton identification
Period Stat ?Mtop (GeV) Stat ??/?
1 year 0.1 0.2
1 month 0.2 0.4
1 week 0.4 2.5
No background included

• Signal can be improved by kinematic constrained
  fit
• Assuming MW1MW2 and MT1MT2

28
Commissioning the detectors

• Most important background for top W4 jets
• Leptonic decay of W, with 4 extra light jets
• Alpgen, Monte Carlo has hard matrix element for
  4 extra jets(not available in Pythia/Herwig)

ALPGEN W4 extra light jets Jet PTgt10, ?lt2.5,
?Rgt0.4 No lepton cuts Effective ? 2400 pb

• Signal plus background at initial phase of LHC

L 150 pb-1 (2/3 days low lumi)
With extreme simple selection and reconstruction
the top-peak should be visible at LHC
measure top mass (to 5-7 GeV) ? give feedback on
detector performance
29
Top in DC2 Tier test

• The 10M tier1 events in light of top
• Generation/simulation of 106 top events,
  inclusive decays, using MC_at_NLO
• Using Herwig for MC UE
• Simulation with full geometry
• Simulation 500K top events with displaced ID
• Same truth generated top events as above
• 1 cm displacement of ID check tracking
  performance
• Simulation of 106 Wjet events MC_at_NLO
• For W2 jet background
• Simulation of 250K W4jet events with AlpGen
• pTgt15 GeV approximately

30
What is still missing?

• Top production is over-weighted in 10M sample
• Unrealistic to ask for more in this sample
• One of priorities in post-production
• Regenerate half of the top MC_at_NLO sample
• Now using Jimmy UE (much more activity), after
  tuning
• Still on the wish-list
• Top events with spin correlations
• TopRex available, perhaps AcerMC?
• Single top events
• also TopRex
• Dedicated samples single- and dilepton top events
• Top events with PYTHIA (cross check with DC1)

31
Conclusions
LHC is top factory ?(tt)830 pb-1 107 events in
first year

• Precise determination of Mtop is waiting
• Challenge to get ?Mtop 1 GeV
• Confirmation that top-quark is SM particle
• Measure Vtb, charge, CP, spin, decays
• Top quarks for commissioning the detectors
• Top peak should be visible with eyes closed
• Todays signal, tomorrows background
• Top quarks as main background for many new
  physics channels

32
Rare SM top decays

• Direct measurement of Vts, Vtd via decays t?sW,
  t?dW
• Decay t?bWZ is near threshold
• (mtMW MZmb) ?
• BRcut(t ?bWZ) ? 6?10-7
• (cut on m(ee) is 0.8 MW)
• Decay t?cWW suppressed by GIM
• factor BR(t ?cWW)
  1?10-13
• If Higgs boson is light t?bWH
• FCNC decays t?cg, t?c?, t?cZ (BR 5?10-11 ,
  5?10-13 , 1.3?10-13 )
• Semi-exclusive t-decays t?bM
• (final state 1 hadron recoiling against a
  jet
• BR(t ?b?) ? 4?10-8, BR(t ?bDs) ? 2?10-7)

33
Top mass from di-leptons

• Use the events where both Ws decay leptonically
  (Br5)
• Much cleaner environment
• Less information available due to two neutrinos
• Sophisticated procedure for fitting the whole
  event, i.e. all kinematical info taken into
  account (cf D0/CDF)
• Compute mean probability as function of top mass
  hypothesis
• Maximal probability corresponds to top mass

Source of uncertainty Di-lepton ?Mtop (GeV)
statistics 0.3
b-jet scale 0.6
b-quark fragm 0.7
ISR 0.4
FSR 0.6
pdf 1.2
Total 1.7
80000 events ?(tt) 20 S/B 10
Selection 2 isolated opposite sign leptons Ptgt35
and Ptgt25 GeV 2 b-tagged jets ETmissgt40 GeV
Mean probability
mass
34
Top mass from hadronic decay

• Use events where both Ws decay hadronically
  (Br45)
• Difficult jet environment
• ?(QCD, Ptgt100) 1.73 mb
• ?(signal) 370 pb
• Perform kinematic fit on whole event
• b-jet to W assignment for combination that
  minimize top mass difference
• Increase S/B
• Require pT(tops)gt200 GeV

Selection 6 jets (?R0.4), Ptgt40 GeV 2 b-tagged
jets Note Event shape variables like HT, A, S,
C, etc not effective at LHC (contrast to Tevatron)
Source of uncertainty Hadronic ?Mtop (GeV)
Statistics 0.2
Light jet scale 0.8
b-jet scale 0.7
b-quark fragm 0.3
ISR 0.4
FSR 2.8
Total 3.0
3300 events selected ?(tt) 0.63
?(QCD) 210-5 S/B 18
35
High Pt sample

• The high pT selected sample deserves independent
  analysis
• Hemisphere separation (bckgnd reduction, much
  less combinatorial)
• Higher probability for jet overlapping
• Use all clusters in a large cone ?R0.8-1.2
  around the reconstructed top- direction
• Less prone to QCD, FSR, calibration
• UE can be subtracted

Mtop
Statistics seems OK and syst. under control
?R
36
Jet scale calibration

• Calibration demands
• Ultimately jet energy scale calibrated within 1
• Uncertainty on b-jet scale dominates ?Mtop light
  jet scale constrained by mW
• At startup jet-energy scale known to lesser
  precision

Uncertainty On b-jet scale Hadronic
1 ? ?Mt 0.7 GeV 5 ? ?Mt
3.5 GeV 10 ? ?Mt 7.0 GeV
Uncertainty on light jet scale Hadronic
1 ? ?Mt lt 0.7 GeV 10 ?
?Mt 3 GeV
37
Rare decays top?WbZ

• Interesting branching ratio depends strongly on
  Mtop
• Since MtopMWMbMZ
• With present error ?mt ? 5 GeV, BR varies over
  a factor ? 3
• B-jet too soft to be efficiently identified ?
• ? semi-inclusive study for a WZ near
• threshold, with Z ? ll- and W -gtjj
• Requiring 3 leptons reduces the Zjets
  background
• Sensitivity to Br(t ? WbZ) ?? 10-3 for 1 year at
  low lumi.
• Even at high L cant reach Sm predictions (??
  10-7 - 10-6)

G. Mahlon hep-ph/9810485
G(t?WbZ)/G(t?Wb)
M(top) (GeV)
38
top?Hq

• Various approaches studied
• Previously ttbar?Hq Wb?(b-bbar)j(l?b) for m(H)
  115 GeV
• Sensitivity to Br(t ? Hq) 4.5 X 10-3 (100
  fb-1)
• New results for
• t tbar?Hq Wb?WWq Wb?(l? l?j) (l?b)
• 3 isolated lepton with pT(lep) gt 30 GeV
• pTmiss gt 45 GeV
• 2 jets with pT(j) gt 30 GeV,
• incl. 1 jet con b-tag
• Kinematical cuts making use of
• angular correlations
• Sensitive to Br(t ? Hq) 2.4 X 10-3
• for m(H) 160 GeV (100 fb-1)

39
Non-SM Decays of Top

• 4thfermion family
• Constraints on ?Vtq?relaxed
• Supersymmetry (MSSM)
• Observed bosons and fermions would have
  superpartners ?
• 2-body decays into squarks and gauginos (t ?
  H b )
• Big impact on 1 loop FCNC
• two Higgs doublets
• H? LEP limit 77.4 GeV (LEP WG 2000)
• Decay t ? H b can compete with t ? W b
• 5 states (h0,H0,A0,H,H-) survive after giving W
  Z masses
• H? couples to heaviest fermions ? detection
  through breakdown of e / m / t universality in
  t?t production